CVE-2016-3159 in Xen
Summary
by MITRE
The fpu_fxrstor function in arch/x86/i387.c in Xen 4.x does not properly handle writes to the hardware FSW.ES bit when running on AMD64 processors, which allows local guest OS users to obtain sensitive register content information from another guest by leveraging pending exception and mask bits. NOTE: this vulnerability exists because of an incorrect fix for CVE-2013-2076.
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Analysis
by VulDB Data Team • 07/11/2022
The vulnerability identified as CVE-2016-3159 represents a critical flaw in the Xen hypervisor's handling of floating-point unit operations on AMD64 processors. This issue specifically affects Xen versions 4.x and stems from improper management of the FSW.ES bit within the fpu_fxrstor function located in arch/x86/i387.c. The flaw enables local guest operating system users to extract sensitive register information from other guest environments through manipulation of pending exception and mask bits. This vulnerability is particularly concerning because it was introduced as an incorrect fix for a previous vulnerability, CVE-2013-2076, creating a regression that undermines security measures previously implemented.
The technical exploitation of this vulnerability occurs through the improper handling of the FSW.ES bit, which is part of the x87 floating-point status word register. When guest operating systems execute floating-point operations on AMD64 processors, the hypervisor must correctly manage the state transitions and ensure proper isolation between virtual machines. The flaw allows malicious guest users to craft specific floating-point instructions that can leak information from other guest VMs by manipulating the exception state bits. This occurs because the fpu_fxrstor function fails to properly validate or sanitize the hardware FSW.ES bit during context switching operations, enabling information disclosure across guest boundaries.
From an operational perspective, this vulnerability fundamentally undermines the isolation guarantees that hypervisors are designed to provide. The ability to extract sensitive register content from other guests represents a significant compromise in virtual machine isolation, potentially allowing attackers to access confidential data, credentials, or system information belonging to other virtualized environments. The impact extends beyond simple information disclosure as it can facilitate further exploitation attempts, including privilege escalation or lateral movement within virtualized environments. This vulnerability particularly affects cloud computing environments where multiple tenants share the same physical infrastructure, as it enables one tenant to potentially compromise another's guest operating system.
The security implications align with CWE-200, which addresses "Information Exposure," and can be mapped to ATT&CK techniques such as T1059 for command and control operations and T1068 for local privilege escalation. Organizations running Xen hypervisors must implement immediate mitigation strategies including applying the appropriate security patches, disabling unnecessary floating-point operations in guest environments, and implementing network segmentation to limit potential attack vectors. The vulnerability also highlights the importance of thorough regression testing when implementing security fixes, as the incorrect solution for CVE-2013-2076 created this new security weakness. System administrators should monitor for signs of exploitation attempts and consider implementing additional logging mechanisms to detect anomalous floating-point operation patterns that might indicate exploitation attempts.